Method of thermal treatment



Patented July 10,1945

'UNlTED STATES PATENT OFFICE METHOD OF THERMAL TREATMENT Philip T. Stroup, New Kensington, Pa., assignor to Aluminum Company-of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application August 31, 1942, Serial No. 456,748

Claims. (01. 148-213) This invention relates to the art of thermal treatment of objects madeof magnesium or its alloys, by which latter term is meant those magnesium alloys containing about 70 percent by weight or more of magnesium, the balance being other elements which have been added to the magnesium to add to, enhance, modify, or change its basic properties. Magnesium and its alloys are peculiarly susceptible to deterioration during thermal treatment. While the rate and extent of the deterioration may be affected by the temperature of treatment, the time at temperature,

- the humidity of the atmosphere, and other facrioration. The effect is not always evidenced by surface discoloration, or other obvious change in surface properties, but will in some instances consist of a decrease in physical properties, such as strength or elongation, or in some chemical. property, such as resistance to corrosion. Therefore, the operator may have no immediate indication that the thermal treatment is producing an inferior product.

Thermal treatments are necessary steps in the production of many products made of magnesium or of its alloys. Whether the desired product be produced by casting, by deformation under applied force, or by other methods, thermal treatments play an important, and often necessary role. A cast or worked product is sometimes submitted to treatment at a determined elevated temperature for the purpose of increasing strength, ductility, stability, corrosion resistance, or other property. A cast billet, slab, or ingot is often heated or preheated prior to being deformed by familiar operations such as rolling, forging, extrusion, and the like. Such working operations are often interrupted so that th partially deformed or worked product can be annealed, i. e., heated to develop in the metal the ductility necessary to further deformation.

metal objects for'the purpose of developing desirable properties or of curing defects. The possibility, if not the fact, of deterioration arises when these thermal treatments are applied to magnesium and its alloys.

The primary object of this invention, whatever its other objects and incidental advantages, is to minimize or prevent deterioration of magnesium and its alloys during thermal treatment thereof by improvements which are adaptable to large scale production.

The present invention includes, as its broadest aspect, the thermal treatment of magnesium and its alloys as above defined, in an atmosphere into which a fluorine-containing substance has been introduced in the form of a vapor. The resultant atmosphere exerts an effect during the thermal treatment which reduces the tendency of magnesium and its alloys -to deteriorate and thus minimizes to a marked extent the possibility that the thermal treatment will seriously impair the metal subjected to it. In its narrower and preferred form, the invention consists in increasing or activating the beneficial effect of this fluorinecontaining atmosphere by incorporating into the magnesium or an alloy thereof an amount of either or both of the elements calcium or beryllium. The mechanism or reaction by which the beneficial effect of the fluorine-containing atmosphere is increased by the presence, in the metal undergoing heat treatment, of either or both of these elements is not known, but the effect produced is substantial and definite to the end that the tendency of the magnesium to deterioration is suppressed, and the risk that thermal treatment will cause unsatisfactory results is very greatly minimized if not eliminated.

Whether the broad or the narrower aspect of the invention be practiced, a'fluorine-containing substance in vapor form is introduced into the atmosphere which surrounds the magnesium, or alloy thereof, during thermal treatment; The usual atmosphere in commercial operations is air sometimes containing small amounts of sulphur dioxide or combustion products and fur ther contaminated, of course, by industrial gases,

These are but instancesof that large class of op- .eratlons which involve the application of heat to moisture, and the like. But the atmosphere may be some inert gas or even a partial vacuum in which residual gas is present. Where considerations of cost do not interfere, it is advantageous to use an atmosphere of carbon dioxide, preferably dry, and this is a preferred step in the practice'of this invention. Whatever the atmosphere (by which is meant the medium surrounding the metal object beingtreated) ,it should contain, in

boron fluoride, or other gaseous fluorides, including the organic fluorides, may be separately generated and introduced into the atmosphere. A i

convenient method consists in heating solid fluoride salts or salts containing fluorine, such as sodium or potassium fluoborate, to produce the required vapor. Where the temperatures employed during thermal treatment are sufllciently high, the fluorine salt may be introduced into the furnace or.other thermal treatment chamber and the temperature therein relied upon to generate from the salt the fluorine-containing substance vapor. These methods are only indicative, and other methods of introducing the fluorine-containing substance in vapor form into the atmosphere surrounding the magnesium during ther-' mal treatment will be apparent to those skilled in the art.

To prepare for the practice of the narrower and preferred aspect of the invention, it is necessary to add to the magnesium, or alloy there ,of, which is later to undergo thermal treatment an amount of either one or both of the elements beryllium and calcium. Such additions are conveniently made to the metal at any time prior to the thermal treatments here described. It is'the presence of these elements in material amount in the magnesium or alloy thereof which increases the effectiveness of the fluorine-containing atmosphere to prevent deterioration during thermal treatment. Therefore, so long as the element, or elements, is effectively present, the exact amount of the element is not important to the precepts of this invention. However, the functions and properties of the final magnesium or magnesium alloy product must be considered and, therefore, it is desirable generally to limit the amount of beryllium or calcium, .or the total of these elements, to that which is effective for the purpose here under discussion. Further amounts may. 0f course, be added for other pur- D Calcium-When used in the absence of beryllium, calcium in amounts as low as 0.01 per cent by weight of the total metal is efiectiv to increase the power and effect of the fluorine-containing substance in--the thermal treatment atmosphere. Very good results have been obtained by the use of 0.05 per cent of this element. Generally the amount of calcium present in commercial magnesium alloys is limited to.1.0 per cent or less, often to about 0.2 per cent. I I

Beryllium is effective for the purposes of this invention when present in amounts as low as 0.0005 per cent by weight. For other reasons than those connected with this invention, it may be advisable to limit its use to about 0.0015 per cent by weight of the alloy because of the detrimental eflect of larger amounts in increasing the size of the grain of magnesium or alloys thereof. However, where size of grain is of no importance or is controlled by other constituents of the alloy or by the method of casting used, beryllium may be used in larger amounts, such as 2 percent, although usually it is difficult to hold more than about 0.2 per' cent by weight uniformly in the alloy. Beryllium in amounts of 0.0001 to 0.01 percent is preferred.

As before stated, the tendency of magnesium and its alloys to deterioration during thermal treatment may be suppressed or limited according tothe principles of this invention by the use of a fluorine-containing substance in vapor form in the atmosphere surrounding the metal during the thermal treatment, and indeed in some cases when the tendency to deterioration under the conditions of the thermal treatment is not marked or is merely sporadic, the presence of fluorine-containing substance in the atmosphere may be sufllcient. However, in most instances it is preferred to practice the invention in its narrower aspect and to increase the effectiveness of the fluorine-containing atmosphere by including calcium or beryllium or both in the magnesium. Such practice minimizes the possibilities that variations in the atmosphere will not suddenly cause deterioration. For example,

I in one instance where articles made of a magnesium alloy (of the maghesium-aluminumzinc type) were undergoing thermal treatment in the month of April, the operator was using only that aspect of the invention which consists I of including a fluorine-containing substance in vapor form in the furnace atmosphere, and results were sa isfactory, no deterioration being bon dioxide atmosphere may be completely ef-' fective to prevent deterioration.

It will be understood that the word alloys" as used inthe appended claims refers to alloys containing about 70 per cent or more of magnesium, and that the words "thermal treatment" do not imply the heating of the object made of magnesium or an alloy thereof to temperatures which would cause substantial melting of the a metal,

I claim:

1. In the art of thermaliytreating magnesium .and alloys containing at least '70 per cent by weight of magnesium, the method of minimizing deterioration of the metal during treatment which comprises providing a fluorine-containing substance in vapor form in the atmosphere'surrounding the metal. r

2. In the art of thermally treating magnesium and alloys containing at least 70- per cent by weight of magnesium, the method of minimizng deterioration of the metal during treatment which comprisessurrounding the metal with an atmosphere of carbon dioxide in which is included I in vapor form a fluorine-containing substance.

3. In the art of thermally treating magnesium and alloys containing at least 70 per cent by weight of magnesium, the method of minimizing deterioration of the metal during treatment which includes providing in the metal prior to the treatment beryllium in amounts from about 0.0005 to about 0.2 per cent by weight of the total metal, and surrounding the metal during the treatment with n atmosphere containing fluorine-containing substance in vapor form.

4. In the art of thermally treating magnesium and alloys containing at least 70 per cent by weight of magnesium, the method of minimizing deterioration of the metal during treatment which includes providing in the metal prior to the treatment beryllium in amounts from about 0.0001 to about 0.01 per cent by weight of the total metal, and surrounding the metal during the treatment with an atmosphere containing fluorine-containing substance in vapor form.

5. In the art of thermally treating magnesium weight of magnesium, the method of minimizing deterioration of the metal during treatment which includes providing in the metal prior to the treatment beryllium in amounts from about 0.0001 to about 0.01 per cent by weight of the total metal, and surrounding the metal-with an atmosphere of carbon dioxide in which is included in vapor form a fluorine-containing substance.

8. In the art of thermally treating magnesium and alloys containing at least 70 per cent by weight of magnesium, the method of minimizing deterioration of the metal during treatment which includes providing in the metal prior to the treatment beryllium in amounts not less than about 0.0005 per cent or more than about 2 per cent by weight, calcium in amount not less than about 0.01 per cent or more than about 1.0 per centby weight, and surrounding the metal durand alloys containing at least 70 per cent by the treatment calcium in amounts from about 0.01 to about Org, per cent by weight of the totel metal, and surrounding the metal during the treatment with an atmosphere containing fluofine-containing substance in vapor form:

'7. In the art of thermally treating magnesium and alloys containing at least 70 percent by ing the treatment with an atmosphere containing fluorine-containing substance in vapor form.

' 9. In the art of thermally treating magnesium and alloys containing at least per cent by weight of magnesium, the method of minimizing deterioration of the metal during treatment which includes providing in the metal prior to the treatment beryllium in amounts from about 0.0005 to about 2 per cent by weight of the total metal, and surrounding the metal during the treatment with an atmosphere containing fluofine-containing substance in vapor form.

10. In the art of thermally treating magnesium and alloys containing at least '70 per cent/by weight of magnesium, the method of minimizing deterioration of the metal during treatment which includes providing in the metal prior to the treatment calcium in amounts from about 0.01 to about 0.2 per cent by weight ofthe total metal, and surrounding the metal with an atmosphere of carbon dioxide in which is included in vapor form a fluorine-containing substance.

' PHILIP T. STROUP. 

